Method for maintaining a desired braking torque

ABSTRACT

A stepped braking system is used as a function of the required braking torque in this method for maintaining a desired braking torque in a vehicle equipped with stepped and infinitely-variable permanent braking systems whereby simultaneous activation of the stepped and infinitely-variable braking systems occurs in such manner that the sum of the participations of the different systems provides the required braking torque at any time.

FIELD OF THE INVENTION

The present invention concerns a method for using the wear-free brakingsystem and in particular for maintaining a desired braking torque of amotor vehicle.

BACKGROUND OF THE INVENTION

In the state of the prior art, nowadays and especially in goodsvehicles, permanent braking units (wear-free brake systems) are used torelieve the load on the operating brakes. Permanent braking units canalso improve the economy of goods vehicles by allowing higher averagespeeds (especially on long downhill drives) and by considerably reducingthe wear of the brake linings of the operating brakes.

Engine brake systems are used as permanent brakes which, during a thrustoperation, produce a braking torque that depends on the gear engaged. Inaddition, retarder brakes are known to convert kinetic energy into heatenergy, which differ in their manner of energy conversion in that withhydrodynamic retarders, the energy is converted by fluid friction and,with electrodynamic retarders, by means of a magnetic field. Retardersact as virtually wear-free permanent brakes, especially for goodsvehicles and locomotives, since they have the advantage of convertingthe energy to be braked into heat without wear over long periods oftime.

In hydrodynamic retarders, the energy flow of a fluid is utilized forbraking, the physical action principle corresponding to that of ahydrodynamic clutch with fixed turbine. According to this, ahydrodynamic retarder comprises a rotor in the fluid flow path and astator fixedly attached to the retarder housing. When the retarder isactuated, a quantity of oil corresponding to the desired brakingperformance is introduced into the turbine blade area, which the rotorimpels against the stator so that a braking action is exerted on therotor.

In electrodynamic retarders, on the other hand, the principle used forbraking is that of the action of force in electromagnetic fields. Here,a stator is provided with several energizing coils and attached to thetransmission housing. Air-cooled rotors are also provided on thetransmission-gear side, which are usually connected to the drive shaft.For braking, the energizing coils are supplied with current. As therotors pass through the magnetic field, eddy currents are induced whichimpede the rotary movement of the rotors.

Depending on their arrangement in the drive train, retarders are dividedinto primary and secondary retarders, primary retarders being arrangedon the engine side and secondary retarders on the transmission side. Inthe present state of the art, electrodynamic retarders are mainlyarranged as secondary retarders. This means that primary retardersoperate as a function of the engine speed, while secondary retardersoperate as a function of the speed of the vehicle.

In addition, permanent braking systems are divided into stepped andinfinitely-variable systems; stepped systems are the engine brakes andthe electrodynamic retarders. In contrast to the infinitely-variablesystems, such as hydrodynamic retarders, the braking performance canonly be varied in steps.

Permanent brakes are particularly important in the case when the speedmust be kept constant downhill, but this often entails discomfort forthe driver.

At lower drive shaft speeds, hydrodynamic secondary retarders have theirsystem limits, i.e., the braking torque produce is no longer sufficient.In addition, with hydrodynamic secondary retarders and, as a rule athigh drive shaft speeds, the power is limited or reduced in order toprotect the engine cooling system.

Accordingly, the purpose of the present invention is to indicate amethod for maintaining a desired braking torque with optimum utilizationof the wear-free braking systems of a motor vehicle.

For this, it is proposed to combine the strengths of the availablebraking systems (e.g., hydrodynamic+electrical secondary retarder,hydrodynamic secondary retarder+engine braking). In this way theweaknesses of the available various respective braking systems can becompensated.

SUMMARY OF THE INVENTION

According to the invention, the method proposed here can be used for anyoperation mode, including the regulation of a constant speed on agradient and maintaining a constant braking torque or maintaining aconstant delay.

Thus, for example, the function of maintaining a constant speed on agradient can be implemented using several mutually independent brakingsystems, with stepped and infinitely-variable braking systems acting incombination. The function described can, therefore, be implemented overnearly the entire speed range.

The prerequisite is to know the braking behavior of the differentsystems. For this purpose, the parameters that characterize the systembehaviour of the various braking systems, for example, the performancegraphs of the system, are stored in a memory and the actual brakingbehavior is measured or calculated.

The performance of the permanent braking system can be improved by usingbraking systems with different energy balances, thanks to the methodaccording to the invention

Furthermore, a stepped braking system can also be used in combinationwith an infinitely-variable braking system to perform the function ofmaintaining constant speed on a gradient without compromising comfort.

The combination of braking systems makes it possible to utilize thedifferent strengths of the systems; for example, at the performancelimit of the hydrodynamic secondary retarder the primary retarder or anelectrical secondary retarder can be used additionally.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a vehicle braking systemincluding a conventional braking system and stepped and infinitelyvariable permanent brakes;

FIG. 2 is a flow diagram illustrating operation of the presentinvention;

FIG. 3 is a representation of the function “constant speed on thegradient” with the aid of a combination of different wear-free brakingsystem in accordance with the invention, and

FIG. 4 is a representation of the function “constant braking torque” and“constant retardation” with the aid of a combination of differentwear-free braking systems in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, and described in the Background of the Invention forexample, at least some vehicles 10 to have both conventional brakes 12operating on the wheels 14 and one or more permanent braking units. Asis well known, permanent braking units reduce the wear on conventionalbrakes 12 by providing braking force in certain circumstances, such ason downhill sections of roads, thereby reducing the use wear of theconventional brakes 12.

As described, permanent braking units may typically comprise the engineoperating in an engine braking mode or various forms of retarders. As iswell known, an engine 16 operates as a permanent braking unit whenproviding engine braking, which occurs when the drive train comprisingthe engine 16, the transmission 18 and the driving wheels 14 of thevehicle 10 are engaged, but the engine 16 operates at a speed lower thanthat required to sustain a current speed of the vehicle 10. In thisinstance, the vehicle drive wheels 14 will drive the engine 16, ratherthan the engine 16 driving the wheels 14, so that the engine will exerta braking force on the driving wheels 14 through the transmission 18. Asis well known, the braking torque or force provided by engine braking isdependent on the speed of the engine 16 and upon the gear ratio of thetransmission 18, so that the braking force provided by the engine 16 isgenerally selectable in discrete steps or levels of braking force,generally determined by the transmission ratio and the engine speed.

Retarders are typically of either of two basic types, that is,hydrodynamic retarders or electrodynamic retarders, which exert abraking torque or force on the wheels 14 by a mechanism that dissipatesdrive torque, that is, kinetic energy from the wheels 14, as heatenergy, thereby exerting a braking force on the wheels 14.

In hydrodynamic retarders, the energy flow of a fluid is utilized forbraking, the physical action principle corresponds to that of ahydrodynamic clutch with fixed turbine. According to this, ahydrodynamic retarder comprises a rotor in the fluid flow path and astator fixedly attached to the retarder housing. When a hydrodynamicretarder is actuated, a quantity of fluid corresponding to the desiredbraking performance is introduced into the turbine blade area and therotor impels the oil against the stator so that a braking action isexerted on the rotor.

In electrodynamic retarders, on the other hand, the principle used forbraking is that of the action of force in electromagnetic fields. Here,a stator is provided with several energizing coils and attached to thetransmission housing. Air-cooled rotors are also provided on thetransmission-gear side, which are usually connected to the drive shaft.For braking, the energizing coils are supplied with current and, as therotors pass through the magnetic field, eddy currents are induced whichimpede the rotary movement of the rotors.

It will be recognized from the above brief description of retarders thatunlike engine braking that provides braking force in discrete steps orincrements dependent upon the transmission 18 gear ratio and the enginespeed, the braking force provided by retarders is infinitely variable.It must also be recognized, however, that the nature and characteristicsat least some retarders, such as electrodynamic retarders, allow theretarders to operate as stepped permanent brakes by controlling theelectrical activation current to the electrodynamic retarder in steps orincrements rather than on a continuously variable basis.

According to the present invention, permanent brakes are categorized aseither stepped brakes or infinitely variable brakes and stepped brakesare exemplified by engine braking or by an electrodynamic retarder whileinfinitely variable brakes are exemplified by hydrodynamic retarders orby electrodynamic retarders.

The present invention further categorizes retarder type permanent brakesas either primary or secondary retarders wherein primary retarders arearranged on the input side of the transmission 18, that is, on theengine 16 side of the transmission 18, and secondary retarders arearranged on the output side of the transmission 18. This means that thebraking forces provided by primary retarders are primarily a function ofthe engine speed while the braking forces provided by secondaryretarders are primarily a function of the vehicle speed. Also, it willbe apparent that an engine 16 operating in engine braking mode can onlybe a primary type permanent brake.

As previously stated, the present invention provides a method formaintaining a desired braking torque with optimum utilization of thewear-free permanent braking systems of a motor vehicle. For this, it isproposed to combine the strengths of the available stepped and variablebraking systems, such as a hydrodynamic retarder operating as a variableprimary brake with an electrodynamic retarder operating as a steppedsecondary brake or an engine 16 operating as a stepped primary brake, inengine braking mode, with a hydrodynamic retarder operating as avariable secondary retarder In this way the weaknesses of the availablevarious respective braking systems can be compensated.

Possible arrangements of stepped and infinitely variable permanentbrakes 20 in a vehicle according to the present invention and accordingto the discussions just above are illustrated in FIG. 1. According tothe present invention and as discussed above, a permanent brake 20 maybe either a stepped permanent brake 20S or an infinitely variablepermanent brake 20V, hereafter referred to as a “variable brake” inwhich the S and V suffixes indicate (S)tepped or (V)ariable permanentbrakes.

Whether a given stepped or variable permanent brake 20S or 20V is aprimary stepped or variable permanent brake is indicated by a secondsuffix appended to the reference numeral. e.g. the primary stepped andvariable brakes are thereby designated by the reference numerals 20SPand 20VP while secondary stepped and variable brakes are designated bythe reference numerals 20SS and 20VS.

Lastly, and as also described above, whether a permanent brake 20SP,20VP, 20SS or 20VS is provided by engine braking or is a hydrodynamicbrake or a electrodynamic brake is indicated by a third suffix to thereference numeral in which the third suffix is an M indicating motor(engine) braking, an H indicating a hydrodynamic brake, or an Eindicating an electrodynamic brake.

The method of the present invention for using stepped and infinitelyvariable permanent brakes is illustrated in FIGS. 2. 3 and 4. FIG. 2 isa flow diagram illustrating the basic method of the present inventionfor using a stepping permanent brake and an infinitely variablepermanent brake to obtain a desired braking force. FIG. 3 illustratesthe method with respect to obtaining a desired braking force providing aconstant vehicle speed, as on a gradient, and FIG. 4 illustrates themethod with respect to obtaining a desired braking force that isconstant.

Referring to FIG. 2. in order to obtain a desired braking force andaccording to the present invention, a stepped braking system 20S (suchas an engine brake 20SPM; or electrodynamic secondary retarder 20SSE) isactivated and the difference 22 to achieve the effective necessaryspecified braking torque 24 is made up with an infinitely-variablebraking system 20V (such as a hydrodynamic retarder 20VPH or 20VSH).

According to the invention, the activation of the stepped braking system20S (such as a 20SPM or 20SSE) takes place when the required specifiedbraking torque 24 is at least larger than the step 26A of braking torqueof the stepped braking system 20S. The residual torque difference 22must be large enough for the infinitely-variable system 20V (such as a20VPH or 20VSH) not to be immediately deactivated. For example, if thenecessary total braking torque 24 is larger than the braking torque 26that can be obtained by the step 26B of braking torque of the steppedbraking system 20S, then the stepped braking system 20S increases itsbraking torque to step 2 braking torque 26B and the infinitely-variablebraking system 20V reduces its braking torque 22 by the same amount. Thesame applies for the other steps in the same way.

In this, the stepped braking system can be activated in one or moresteps.

The system limits of the “constant speed on a gradient” function arethus displaced in the direction of lower speeds and a larger speed rangecan, therefore, be covered.

FIG. 3 further illustrates the way in which the method operates with theaid of a braking torque-time diagram.

As a function of the required braking torque M brake 24, a steppedbraking system 20S is used with simultaneous activation of aninfinitely-variable braking system 20V in such manner that the sum ofthe participations of these different systems, at any time t, providesthe required braking torque 24. The step 26 (26A–26D . . . ) of thestepped system 20S to be activated in each case is calculated from thebraking torque required 24 so that the residual torque difference 22 orthe participation of the infinitely-variable system 20V is large enoughfor the infinitely-variable system 20V not to be immediatelydeactivated. This ensures that the infinitely-variable system 20Vcontributes to the braking process to increase the driver's comfort.

To implement the “constant braking torque” or “constant retardation”function, the procedure is as shown in the braking moment-speed diagramof FIG. 4. Here too, the stepped braking system 20S is activated whenthe necessary specified braking torque 24 is at least larger than thefirst step 26 of the stepped braking system 20S. The residual torquedifference 22 must again be large enough for participation of theinfinitely-variable system 20V not to become zero.

Thanks to the method according to the invention, the characteristicfeature of primary retarders to give rise to abrupt braking torquechanges when a gear change occurs can be compensated. It is alsopossible, when changing down in thrust operation to compensate for thelack of braking torque during the gear-shift process by maintaining aconstant braking torque or retardation independently of gear-shiftprocesses.

1. A method for providing a desired braking torque in a vehicle, themethod comprising the step of: determining a desired braking torque forthe vehicle; activating a stepped retarder braking system during brakingof the vehicle wherein the stepped retarder braking system has acapability of providing a plurality of steps of braking torque and astepped braking torque provided by the stepped retarder braking systemis a function of one of engine speed and transmission speed, bycomparing the plurality of steps of braking torque that can be providedby the stepped retarder braking system with the desired braking torqueand determining a step of braking torque which is less than the desiredbraking torque; and activating an infinitely-variable retarder brakingsystem in cooperation with the stepped retarder braking system toprovide a combined braking torque representing a difference between thestep of braking torque provided by the stepped braking system and thedesired braking torque, an infinitely variable braking torque providedby the infinitely variable retarder braking system is a function of oneof engine speed and transmission speed and so that a sum of a brakingtorque of the stepped retarder braking system is continuously combinedwith the braking torque provided by the infinitely-variable retarderbraking system to provide the desired braking torque.
 2. The methodaccording to claim 1, further comprising the step of obtaining thedesired braking torque from one of a “constant speed on a gradient”function, a “constant braking torque” function and a “constantretardation” function.
 3. The method according to claim 1, furthercomprising the step of selecting a braking torque less than the desiredbraking torque to be provided by the stepped retarder braking system sothat a difference between the step of braking torque provided by thestepped retarder braking system and the desired braking torque issufficiently large that the infinitely variable retarder braking systemprovides an infinitely variable braking torque continuously with thestepped braking torque provided by the stepped retarder braking systemduring braking of the vehicle.
 4. A method for maintaining a desiredbraking torque with optimum utilization of wear-free braking systems ofa vehicle, the method comprising the step of: combining in the vehicledrive train a stepped retarder braking system and an infinitely-variableretarder braking system, wherein a stepped braking torque provided bythe stepped retarder braking system and an infinitely variable brakingtorque provided by the infinitely variable braking system are eachdependent upon one of engine speed and transmission speed; determining adesired braking torque for the vehicle; activating the stepped retarderbraking system during braking of the vehicle; wherein the steppedbraking system has a capability of providing a plurality of steps ofbraking torque, by comparing the plurality of steps of stepped brakingtorque that can be provided by the stepped braking system with thedesired braking torque and determining a step of braking torque which isless than the desired braking torque; and activating theinfinitely-variable braking system in cooperation with the steppedretarder braking system to provide a second braking torque representinga difference between the step of braking torque provided by the steppedbraking system and the desired braking torque so that a sum of a steppedbraking torque of the stepped braking system is continuously combinedwith the second braking torque provided by the infinitely-variablebraking system to provide the desired braking torque.
 5. The methodaccording to claim 4, further comprising the step of obtaining thedesired braking torque from one of a “constant speed on a gradient”function, a “constant braking torque” function and a “constantretardation” function.
 6. The method according to claim 4, furthercomprising the step of selecting a braking torque less than the desiredbraking torque to be provided by the stepped retarder braking system sothat a difference between the step of braking torque provided by thestepped retarder braking system and the desired braking torque issufficiently large that the infinitely variable retarder braking systemprovides an infinitely variable braking torque continuously with thestepped braking torque provided by the stepped retarder braking systemduring braking of the vehicle.